The existence of substances able to change colour at a predetermined temperature is known. Said substances are referred to as being “thermochromic”. Generally, said substances form part of the category of liquid crystals.
During the last few decades numerous inks based on thermochromic substances have been investigated. These inks are called “thermochromic inks” and are used for silk-screen printing, flexographic printing, wet offset printing, lithographic printing and the like.
Some of these inks are coloured and change colour at a predetermined temperature. Other thermochromic inks are colourless and become coloured at a predetermined temperature. There are also other inks which are coloured and become colourless at a predetermined temperature.